This invention relates to optical routing devices and, more particularly, to optical devices that perform an add-drop multiplexing function.
The information superhighway will primarily comprise optical fibers for the foreseeable future because of the enormous bandwidth that each optical fiber provides. For example, an optical fiber exhibits relatively low loss over the wavelength region 820-1600 nm. This particular region provides a bandwidth of about 180,000 GHz which means that a single optical fiber can potentially carry 45 billion voice channels (4 kHz each) or 30 million television channels (6 MHz each). And while these numbers represent upper limits that are not practical to attain, they provide a compelling reason for communication carriers to use optical transmission.
However, in order to fully utilize this information superhighway, there needs to be convenient equipment for re-directing (routing) individual optical channels, or groups of channels, at multiple intermediate locations along an optical transmission path. Optical routers perform this function, and generally comprise multiplexers and demultiplexers. One device, which is known as an add-drop multiplexer (ADM), comprises a demultiplexer having output ports that are connected to the input ports of a multiplexer by waveguides, some of the waveguides including a multi-port optical switch for adding and/or dropping a channel from the optical transmission path.
FIG. 1 shows a prior-art ADM 10 that services sixteen channels operating at different nominal wavelengths (xcex1, . . . , xcex16). Demultiplexer 300 separates a multiplexed optical signal, which is present on input waveguide 101, into its component channels and makes them available on output ports 201-216; whereas multiplexer 400 combines input channels on ports 301-316 into a composite output signal for transmission on waveguide 401. Switches 20-1 through 20-16 are connected in each of the waveguides that extend between the demultiplexer and the multiplexer, and route optical signals on input ports A, B to output ports C, D. More will be said later regarding the operation of this ADM, but what is important to note at this time is the waveguide crossings associated with this structure. Unfortunately, such crossings cause increased crosstalk and insertion loss. Not surprisingly, the magnitude of crosstalk and insertion loss increase in direct proportion to the number of waveguide crossings. It is estimated that optical loss in an amount of 0.1 dB per waveguide crossing is introduced. Moreover, because of scattering, the same approximate amount of crosstalk is introduced by each waveguide crossings.
FIG. 2 discloses a prior-art ADM 20 that avoids waveguide crossings, but which is somewhat more expensive than the ADM 10 of FIG. 1 because it requires a circulator. Moreover, as a practical matter, the ADM 20 is limited to about four channels. Bragg reflectors are serial devices that cause the ADM to become prohibitively long and loss when many channels are involved. More will be said later regarding the operation of ADM 20.
Accordingly, what is desired is an ADM that is capable of handling many channels with lower signal loss and crosstalk than has been achieved with prior-art devices.
An optical ADM that overcomes the problems of the prior art is constructed as a Mach-Zehnder interferometer having a multiplexer and a demultiplexer in each arm. Each multiplexer-demultiplexer pair is interconnected by coherent connecting paths (i.e., all paths preserve polarization and have the same phase delay xc2x1Nxc3x97180xc2x0). Selected ones of the connecting paths include elements for increasing its effective optical length. One optical coupler is connected to the input ports of the demultiplexers in order to distribute equal amounts of input lightwave signals to the demultiplexers. Another optical coupler is connected to the output ports of the multiplexers in order to recombine the lightwave signals from each arm of the interferometer.
In illustrative embodiments of the invention, the optical couplers are symmetrical waveguide couplers, or adiabatic waveguide couplers, or Y-branch couplers. In order to assure that the connecting paths in each of the interferometer arms are coherent, the entire ADM preferably resides on the same silicon substrate.
In an illustrative embodiment of the invention, thin-film heaters are activated to increase the lengths of the connecting paths. Also in an illustrative embodiment of the invention, each connecting path between the demultiplexer and the multiplexer includes several waveguides with a thin-film heater positioned above the central waveguide(s); but not above the outside waveguides. Advantageously, this provides a guard band between adjacent channels so that when one channel is added or dropped, the adjacent channels experience little or no interference. The end-to-end transmission characteristic of the ADM through each individual waveguide has a Gaussian shape. These Gaussian shapes are arranged to intersect at wavelengths corresponding to their xe2x88x923 dB (half-power) levels so that the end-to-end transmission characteristic of the ADM is substantially flat. Accordingly, flat passbands for pass-through channels and flat passbands for add-drop channels can be achieved in an ADM without intrinsic loss.